This invention relates to the treatment and disposal of acidic photoresist resins after development and stripping during manufacture of printed circuit boards and the like. More particularly, it relates to the neutralization of aqueous alkaline solutions of resins which result from such operations. Still more particularly, it relates to the fast filtration of the sludge created by the neutralization and acidification of such solutions.
In the preparation of printed circuit boards, a copper-clad substrate is coated with an alkali-soluble photo-sensitive polymer called a photoresist and the photoresist is exposed image-wise to actinic radiation to further polymerize the exposed areas and make them less soluble. The negative image is that developed on the substrate by dissolving away the non-exposed areas while leaving the less soluble resin in place. The copper that is thus uncovered is removed by an etchant and then the remaining photoresist is stripped from the substrate by another alkaline solution that is suitably hot. The development and stripping of positive photoresists are done in aqueous alkaline solutions in like manner.
The advantages of forming resinous photopolymerizable compositions which can be developed in aqueous systems without the need of organic solvents have been recognized as far back as 1956. After several years of attempts by many investigators in the field of photoresist chemistry, Gilano et al. disclosed alkali-soluble photoresists having a binder comprising a co-polymer of styrene or other vinyl monomer and an unsaturated carboxyl-containing monomer in U.S. Pat. No. 3,953,309. The teachings of Gilano et al. in the '309 patent are incorporated herein by reference. One of the problems arising from this advance in the art, however, is the disposal of huge volumes of alkaline waste materials which must be neutralized before they may be introduced into sewage treatment plants or into a landfill. Neutralization with an acid would seem to be the most natural and most straightforward way but the resin becomes a sticky precipitate which constantly clogs the systems designed to separate it from the aqueous phase. The use of aluminum sulfate to overcome that problem and produce a particulate, non-sticky precipitate containing the photoresist resin and aluminum hydroxide which may be filtered from the neutral slurry is taught in U.S. Pat. No. 4,857,206, which also is incorporated herein by reference. Although the filtration rate is much better than what had been suffered through before, the volume of the filter cake is so great that the filter press must be opened for removal of the cake with uneconomical frequency. The greater part of this volume is that attributable to the bulky aluminum hydroxide matrix surrounding the particles of solid resin.
The removal of photopolymer resins from spent developer and stripper mixtures without the formation of an unctuous, gumlike sludge may be accomplished by the addition of an acid, a polyvalent salt, and a coagulation aid to the mixtures, according to U.S. Pat. No. 4,760,014. The ratio of acid to polyvalent cation is from 5:1 to 35:1 by weight.
Aluminum sulfate is also taught as a coagulant for sludges in the treatment of municipal waste water in U.S. Pat. No. 4,028,238. The dewatering of such sludges by filtration is taught therein to be extremely difficult and the solids content of the sludge after filtration is said to be less than 15% by weight. The solution to the problem, according to the '238 patent, is to add calcium hydroxide to the sludge after the alum treatment and decanting of the supernatant water. The rate of filtration is increased seven-fold and the solids content is markedly increased.
In U.S. Pat. No. 1,173,698, titanium sulfate is used to facilitate the precipitation of aluminum hydroxide which occurs when aluminum sulfate is added as a coagulant to certain water supplies. Titanium hydroxide is also formed and the patentee teaches that it induces a more rapid and more finely divided precipitation of the aluminum hydroxide.